Aircraft in engine powered flight, especially those flying in clouds of dust, rain or ice, often become highly charged electrically. This can cause operational problems. For example, for a helicopter crew hovering over a dusty surface to attempt to lift objects off the ground requires that a crewman grab a metallic cable hanging from the helicopter and attach it to ground-based objects. The ground crewman often receives a severe electrical shock upon his making contact with the helicopter cable. Attempts to eliminate this hazard by use of non-conducting hoist cables have not been acceptable in many applications because the lifting attachment, such as a hook on the lower end of the cable, must be controlled remotely by the aircraft pilot, and this requires the use of conducting wires from the fuselage to the hook for signals and acuating power. Attempts to ground the helicopter before direct contact by the ground crew also have not been successful in all operations due to the difficulty of getting a good ground connection, and because of the operational difficulties with cables swinging violently in the downwash of air beneath the helicopter.
Similarly, with fixed wing airplanes flying through thunderclouds, ice crystals, or rain, the charging often becomes so great that electrical discharges into the surrounding air occur, producing radio-frequency noise which interferes with necessary external communications. The charging of research aircraft makes difficult any measurements of natural electrical conditions inside thunderclouds, and can often cause the taking of erroneous data.
For these reasons, an acceptable means of controlling the charge on an aircraft in flight has been greatly needed.
U.S. Pat. No. 3,035,208 to Clark shows a system directed towards this end. Clark notes that, in good weather, engine exhaust is often capable of maintaining aircraft electrically discharged, or at a small charge. Clark discloses a system having a detector to determine the polarity of charge on the aircraft, and an electrode exposed to the stream of exhaust gas from the aircraft's engine to collect ions from the gas stream. A controller in the system sets the polarity on the electrode so that ions that are collected are of the correct polarity to discharge the aircraft. Another electrode of opposite polarity may be added to deflect additional charge towards the first electrode, making this charge collection more efficient.
In addition to Clark's observations, it is today generally recognized that an isolated aircraft in flight becomes electrified principally as a result of elastic collisions involving contact electrification with atmosphereic particles, by inductive effects arising when water drops leave the aircraft surface in an electric field, and currents flowing in the hot exhaust gases from the engine under the influence of local electric fields.
We have found that this last effect is dominant for the charging of hovering helicopters, whether in dust clouds or in the absence of dust or aerosols (e.g. helicopters hovering over ships at sea in good weather), and we have found that the best way to minimize this charging is to control the electric fields acting on the hot exhaust gases. Although our experimental work has been generally limited to helicopters, we believe that similar charging phenomena on other kinds of aircraft similarly result predominantly from local electric fields operating on exhaust gases expelled by the aircraft, and that control of these external electric fields would similarly best control unwanted aircraft charging.
In particular, these local, external, electric fields cause charge separation much as result from the charge on the electrodes described in Clark's patent. Charges of the same polarity as the external field are deaccelerated and remain in the vicinity of the aircraft, where a portion of these charges accumulate on the exhaust stack and engine walls, charging the aircraft. Charges in the exhaust gas opposite in polarity to the external electric field are accelerated away from the aircraft, and, either have no effect, or, in the case of a hovering helicopter are caught in the downdraft of the helicopter's rotors and swept downward and away. The removal of these charges that are opposite in polarity to the external fields in the vicinity of the exhaust stack in effect increases the external electric field there, reinforcing the basic cause of the helicopter's charging, worsening it. Aerosols or dust provide an excellent medium for transporting these downward charges, and their presence exacerbates this unwanted positive charging feedback.
As examples of this mechanism, we have measured electrical currents in excess of 300 micramperes flowing in tested exhaust gases when we applied strong electric fields to them, and we have controlled the charge on an isolated jet engine merely by placing a charged piece of plastic near, but outside, the exhaust plume. The electric fields from the charge on the plastic caused ions of one polarity to be selectively carried away from the engine by the exhaust gases, and causes it to be highly charged with the same polarity as the plastic.
As another example, for a helicopter disposed on an electrically isolated stand, by electrically shielding the two exhaust stacks from local charges and from charges emitted in the escaping exhaust plume, we have caused the helicopter to charge to potentials in excess of 150 kV with the application of potentials of only 10kV to an electrode mounted in each of its exhaust stacks.